1. Field of the Invention
The present invention relates to an OLED panel in which an organic light emitting device (OLED) formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to an OLED module in which a controller IC or the like is mounted on the OLED panel. Note that, in this specification, the OLED panel and the OLED module are generically called light emitting devices. The present invention further relates to an electronic device using the light emitting device.
2. Description of the Related Art
An OLED emits light by itself, and thus, has high visibility. The OLED does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the OLED has no limitation on a viewing angle. Therefore, the light emitting device using the OLED has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode/a light emitting layer/a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode/a hole injecting layer/a light emitting layer/a cathode are laminated in order or a structure in which an anode/a hole injecting layer/a light emitting layer/an electron transporting layer/a cathode are laminated in order.
In putting a light emitting device to practical use, a serious problem at present is a reduction in the luminance of the OLED, which is accompanied with deterioration of the organic light emitting material contained in the organic light emitting layer.
The organic light emitting material in the organic light emitting layer is easily affected by moisture, oxygen, light and heat, and the deterioration of the organic light emitting material is promoted by these substances. Specifically, speed of the deterioration of the organic light emitting layer is influenced by a structure of a device for driving the light emitting device, a characteristic of the organic light emitting material constituting the organic light emitting layer, a material for an electrode, conditions in a manufacturing process, a method of driving the light emitting device, and the like.
Even when a constant voltage is applied to the organic light emitting layer from a pair of electrodes, the luminance of the OLED is lowered due to the deterioration of the organic light emitting layer. Then, if the luminance of the OLED is lowered, an image displayed on the light emitting device becomes unclear. Note that, in this specification, a voltage applied to the organic light emitting layer from one pair of electrodes is defined as an OLED driving voltage (Vel).
Further, in a color display mode in which three kinds of OLEDs corresponding to R (red), G (green) and B (blue) are used, the organic light emitting material constituting the organic light emitting layer differs depending on the corresponding color of the OLED. Therefore, there may be the case where the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors. In this case, the luminance of the OLED differs depending on the color with the lapse of time. Thus, an image having a desired color can not be displayed on the light emitting device.
Furthermore, the luminance of the OLED has large temperature depending property, and thus, there is a problem in that luminance of a display and a tone vary in accordance with the temperature in constant voltage drive.